Regulation of Phialide Morphogenesis in Aspergillus nidulans

Filamentous fungi have two distinctive life cycles, vegetative growth and development for sexual or asexual spore formation. The asexual reproduction in development as conidiation in A. nidulans is the dominant form of producing spores effectively. A complex conidiophore structure is developed during asexual reproduction process. The conidiophore is formed from hyphal cell and consists of stalk, vesicle, metulae, phialide and conidial spores. Phialides are essential sporogenous cells in the conidiophore structure. The growth pattern is switched from acropetal to basipetal between phialide and spores, which makes phialide a unique cell type in A. nidulans and other phialide producing fungi. Study of the phialide morphogenesis would provide significant insight into the morphogenesis variation in fungi and animals. AbaA is the key transcriptional factor that controls phialide formation, but the knowledge to genes which are directly regulated by AbaA and involved in phialide morphogenesis is poorly known. In this study, twelve genes that are up-regulated by AbaA and potentially related with phialide morphogenesis were selected by gene screening with several criteria we set up, including homology search against other ascomycetes and conserved domain search. The RT-PCR result confirmed that the expression of these genes are induced during developmental stage, probably by AbaA as they contain AbaA binding sites. We also characterized the functions of these genes by generating gene deleted mutants. Two genes have been identified to regulate the proper function of phialide: ndrA (AN11101) and phiB (AN0499). A. nidulans Axl2 may regulate the expression of these two genes in development. They may also play roles as the marker for morphogenetic machinery repositioning during conidiation. Other genes also show relationship to phialide morphogenesis since their mutants exhibited defects in conidiophore